A telescopic sight, also known as a scope or zoom scope, is a sighting device that is based on an optical refracting telescope. Telescopic sights are equipped with some form of graphic image pattern, referred to as a reticle, mounted in an optically appropriate position to provide an accurate aiming point. Telescopic sights are used with a variety of types of systems that require accurate aiming and are commonly found on firearms, e.g., rifles in particular.
Telescopic sights based on refracting telescopes using image erector lenses to present the user, or operator, with an upright image typically have two planes of focus where a reticle can be placed—at the focal plane between the objective and the image erector lens system or the focal plane between the image erector lens system and the eyepiece set. A typical telescopic sight has a circular actuation mechanism to adjust the zooming power. However, there are some drawbacks associated with the circular actuation mechanism. Firstly, a clearance cut on the scope tube of the telescopic sight significantly undermines the structural strength of the telescopic sight. More importantly, the motion of turning the zoom ring of the telescopic sight circumferentially would first cause movement of the firearm on which the telescopic sight is mounted, such that the user would temporarily lose the acquisition of the target. Secondly and most importantly, when adjusted from the minimum to the maximum zooming power, some telescopic sights would require more than one turning action by the user to achieve such power change (also known as long throw adjustment). In combat actions and in some shooting sports, such actuation would negatively impact the firearm user in acquiring the target quickly and shooting effectively.